Plurality voting, where each voter casts a vote for one candidate and the candidate with the most votes wins, even if it's less than a majority.

Two round runoff, where each voter casts a vote for one
candidate, and a runoff is held between the top two candidates if no
candidate receives a majority (or some other threshold, such as 40%) of
the votes cast.

Fairvote supports the use of instant runoff voting for
single-seat offices in public elections. Voters rank candidates in
order of choice, winning requires a majority of the votes, and
candidates are successively eliminated and ballots are recounted if no
candidate receives a majority in a round.

A Borda Count is the technical name for the voting system in
which a first place vote is worth 4 points, a 2nd is worth 3 points, a
3rd 2 points and a 4th is worth 1 point. The candidate receiving the
most points wins. The Borda Count is often used to rank sports teams or
to induct athletes into halls of fame. One problem with the Borda
Count is that ranking a less preferred choice will count againt your
favorite choice.

In Approval Voting, each voter can approve of (vote for) as many
candidates as she supports. The candidate with the greatest approval
wins. Approval voting only measures whether or not a candidate is
acceptable to the voter; it does not distinguish between a candidate
who is intensely liked - a first choice - and those who are more weakly
approved of -- second and lower choices. While simple in design,
approval voting creates incentives for complex campaign strategies.It also could result in the defeat of a candidate whom an absolute majority support as their first choice.

The Condorcet rule elects the candidate who can top each of the
others in a series of head-to-head contests. If most voters prefer
(rank) A over B, A wins that contest. The rankings are used to
determine the winner of each possible head-to-head contest. The
Condorcet rules suffers from the Condorcet Paradox: there may not
be any candidate who defeats all the others: A might beat B, B
might beat C, and yet C could beat A. In this case, some other
system must be used to resolve the paradox. In addition, the Condorcet
candidate might be one with so little core support that he or she
would never have been able to win under any of the single-winner voting
systems currently used for all governmental elections in the United
States and other nations.